The present invention relates to photomasks which are used to produce integrated circuits of high integration density, e.g., large-scale integrated circuits (LSI), very large-scale integrated circuits (VLSI), ultra large-scale integrated circuits (ULSI), etc. More particularly, the present invention relates to an overlying shifter type phase shift photomask which is used to form fine-line patterns on a wafer at high density by projection exposure. The present invention also relates to a blank which is used to produce such an overlying shifter type phase shift photomask.
With the achievement of high integration density of semiconductor integrated circuits, reticles which are used to produce integrated circuits of high integration density have also been required to further reduce line width At present, the line width of device patterns transferred from 5.times.reticles for 16M-bit DRAMs is as fine as 0.6 .mu.m. In the case of the device patterns of 64M-bit DRAMs, line width resolution of 0.35 .mu.m is required. Accordingly, the demanded device patterns cannot be realized by the conventional photolithographic method that employs a stepper because of the pattern resolution limit of this method. Under these circumstances, attention has been given to a phase shift photomask which enables an improvement in resolution of device patterns transferred from a reticle in a photolithography process and which can be used with the existing stepper.
The basic idea and principle of the phase shift photomask have already been disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 58-17344 and Japanese Patent Application Post-Exam Publication No. 62-59296. Recently, the merit of the phase shift photomask constructed from the existing photolithographic system has been recognized anew, and various types of phase shift photomask have been actively developed. One of them is an overlying shifter type phase shift photomask, which has a shifter layer pattern that overlies at least a light-blocking layer pattern formed on a transparent substrate.
Resolution obtained when projection exposure is carried out on a wafer by using the overlying shifter type phase shift photomask will be briefly explained below with reference to FIGS. 7(A) and 7(B) in comparison to a projection exposure process that uses a conventional photomask with no shifter layer.
Part (a) of FIG. 7(A) shows the way in which projection exposure is carried out by using an overlying shifter type phase shift photomask 701. Part (b) of FIG. 7(A) shows the amplitude of light at the exit side of the photomask 701 when projection exposure is carried out as shown in part (a). Part (c) of FIG. 7(A) shows the light amplitude distribution on the wafer. Part (d) of FIG. 7(A) shows the light intensity distribution on the wafer.
Part (a) of FIG. 7(B) shows the way in which projection exposure is carried out by using a conventional photomask 701a. Part (b) of FIG. 7(B) shows the amplitude of light at the exit side of the photomask 701a when projection exposure is carried out as shown in part (a). Part (c) of FIG. 7(B) shows the light amplitude distribution on the wafer. Part (d) of FIG. 7(B) shows the light intensity distribution on the wafer.
The overlying shifter type phase shift photomask 701, which is shown in part (a) of FIG. 7(A), has a transparent substrate 702. Line & space patterns of light-blocking films 703 are formed with a predetermined width and at a predetermined pitch on the transparent substrate 702. A shifter layer 704 is disposed so as to lie over every other opening in the line & space patterns and upon the light-blocking films 703 which are adjacent to the opening. The conventional photomask 701a, which is shown in part (a) of FIG. 7(B), has a transparent substrate 702a on which line & space patterns of light-blocking films 703a are disposed with a predetermined width and at a predetermined pitch.
When exposure light is incident on the overlying shifter type phase shift photomask 701, at the exit side of the photomask 701, the amplitude of light passing through the shifter regions 704 is m.pi. (m is an odd integer) shifted in phase with respect to the amplitude of light passing through the openings defined between the light-blocking films 703 where no shifter is present, as shown in part (b) of FIG. 7(A), thereby effecting phase inversion. Accordingly, the light passing through the shifter regions 704 and the light passing through the openings interfere with each other on the wafer, producing an amplitude distribution such as that shown in part (c) of FIG. 7(A). As a result, the light intensity on the wafer has a distribution such as that shown in part (d) of FIG. 7(A).
In contrast to the above, when the conventional photomask 701a is used, at the exit side of the photomask 701a, there is no phase shift between the amplitudes of light rays passing through the openings, as shown in part (b) of FIG. 7(B). These transmitted light rays interfere with each other, producing an amplitude distribution such as that shown in part (c) of FIG. 7(B). As a result, the light intensity on the wafer has a distribution such as that shown in part (d) of FIG. 7(B).
It will be understood from FIGS. 7(A) and 7(B) that in the case of part (d) of FIG. 7(A), the light intensity is zero at a region between each pair of adjacent crests of the light intensity distribution curve, whereas, in the case of part (d) of FIG. 7(B), the light intensity distribution curve shows an undesirably gentle slope and has no region where the light intensity is zero. With respect to forming the resist pattern resolution on the wafer, the light intensity distribution shown in part (d) of FIG. 7(A) is superior to the light intensity distribution shown in part (d) of FIG. 7 (B).
It is ideal to set the thickness of the shifter regions 704 so that the phase difference is m.pi. (m is an odd integer). With the phase difference of m.pi., the most favorable phase shift effect can be obtained, and in the range of from m.pi.-.pi./3 to m.pi.+.pi./3 (m is an odd integer), resolution can be effectively improved.
It is necessary in order to obtain high resolution to control the phase shift quantity of exposure light at the shifter so that exposure light passing through the openings covered with the shifter is inverted in phase with respect to exposure light passing through the openings where no shifter is present, as has been described above. The phase shift quantity is .phi. [radian] which is expressed by EQU .phi.=2.pi.(n-1)d/.lambda. (1)
where n is the refractive index for the exposure light wavelength of a material used to form the shifter (phase shift layer), d is the thickness of the phase shift film, and .lambda. is the wavelength of exposure light.
The film thickness of the shifter (phase shift layer) is determined by the wavelength of exposure light and the refractive index of the shifter material (phase shift layer material), and the control of film thickness of the shifter (phase shift layer) is important in controlling the phase angle.
Assuming that the exposure light is the i-line (365 nm), and the shifter material is silicon dioxide, since the refractive index of the shifter material (silicon dioxide) is about 1.5, the film thickness required to shift the phase by 180 degrees is nearly 400 nm. It should be noted that silicon dioxide exhibits a refractive index of the order of 1.5 regardless of the film formation method employed.
In general, the light-blocking film is formed from a chromium film. In many cases, the light-blocking film has a triple-layer structure in which anti-reflection films are provided on both sides of a chromium film. The light-blocking film needs a film thickness of the order of 100 nm in order to obtain sufficient light-blocking effect.
In the case of 5.times.reticles used for 64M-bit DRAMs, the pattern size is about from 1.0 .mu.m to 1.5 .mu.m.
Thus, in the case of an overlying shifter type phase shift photomask, a phase shift film of 400 nm in thickness is formed on light-blocking film patterns having a thickness of 100 nm. Therefore, in a conventional manufacturing process shown in FIG. 5, the surface of the shifter (phase shift layer) is affected by the unevenness of the underlying light-blocking film patterns, and the resulting shifter has a cross-sectional configuration such as that shown in FIG. 6(a). Consequently, the phase is disordered within a light-transmitting region, making it impossible to control the phase angle accurately.
The influence on the phase shift angle largely varies according to the pattern spacing, the method of forming the phase shift film, and film forming conditions. This is also a problem to be solved.
An ideal shifter cross-sectional configuration for suppressing the phase disorder is such as that shown in FIG. 6(b) or 6(c).
To improve the cross-sectional configuration of the shifter regions, the shifter layer should be formed flat on the light-blocking film. Based on this idea, the use of a spin-on-glass (SOG) film as a leveling film has been proposed, as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 05-297569.
With the proposed method, however, it is exceedingly difficult to form a leveling film of SOG on a light-blocking layer having recesses and projections, which have a width of 1.0 .mu.m to 1.5 m and a height of about 100 nm, such that the thickness of the leveling film over the recesses is 400 nm. Consequently, the resulting shifter undesirably has a cross-sectional configuration such as that shown in FIG. 6(a). Thus, the phase is disordered in the same light-transmitting region, and the phase angle cannot accurately be controlled.
As has been described above, in the conventional overlying shifter type phase shift photomask, the shifter material is influenced by the unevenness of the underlying light-blocking film patterns, so that the desired cross-sectional configuration cannot be obtained. Consequently, the phase is disordered in the same light-transmitting region of the shifter material, and the phase angle cannot accurately be controlled. Accordingly, it has been demanded to cope with these problems.